Pre-heat the oven to 180°C / 160°C fan assisted / gas mark 4 / 350°F.
Line and grease a 23x33cm (9x13 inch) rectangular cake tin with baking parchment.
In a large bowl or jug, whisk together the eggs, sugar, olive oil, milk and vanilla until smooth and well combined.
In a separate large bowl, sift together the flour, cocoa powder, baking powder and salt and whisk to combine.
Pour the wet mixture into the dry ingredients and mix well to combine.
Pour the chocolatey batter into the prepared cake tin and spread it evenly into the corners.
Bake in the pre-heated oven for 40-45 minutes, or until the sponge is risen, set and a skewer inserted into the centre comes out clean.
Cool in the tin for 10 minutes, then transfer onto a wire rack to cool completely before icing.